evp_keymgmt_util_get_deflt_digest_name() is a refactor of the provider
side key part of EVP_PKEY_get_default_digest_name(), that takes
EVP_KEYMGMT and provider keydata pointers instead of an EVP_PKEY
pointer.

We also ensure that it uses SN_undef as the default name if the
provider implementation gave us an empty string, since this is what
EVP_PKEY_get_default_digest_name() responds when getting the digest
name via a EVP_PKEY_ASN1_METHOD ctrl call that returns NID_undef.
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11576)